Epilepsy-associated increase in gonadotropin-releasing hormone neuron firing in diestrous female mice is independent of chronic seizure burden severity

Abstract

Reproductive endocrine disorders are common comorbidities of temporal lobe epilepsy (TLE). Our previous studies using the intrahippocampal kainic acid (IHKA) mouse model of TLE demonstrated that many females show prolonged estrous cycles and hypothalamic gonadotropin-releasing hormone (GnRH) neurons exhibit elevated firing during diestrus. However, it is unknown whether the degree of change in GnRH neuron activity is dependent on epilepsy severity. Here, we used 24/7 in vivo EEG and in vitro electrophysiological recordings in acute brain slices to assess GnRH neuron firing in relation to chronic seizure burden in diestrous female mice at two months after IHKA injection. We found that percentage of time in seizure activity in the 24 hours prior to slice preparation is an accurate proxy of overall seizure burden. Firing rates of GnRH neurons from EEG-recorded IHKA mice were increased in comparison to controls, but no relationships were found between GnRH neuron firing and seizure burden measured in vivo. The independence of GnRH neuron firing rate in relation to seizure burden was unaffected by GnRH neuron soma location or estrous cycle length. Furthermore, GnRH neuron firing rates were not yet different from control values when measured 1 month after injection when epileptogenesis is already complete in IHKA mice. This result suggests a potential window for intervention to prevent epilepsy-associated GnRH neuron dysfunction. These findings indicate that the severity of epilepsy and the degree of downstream disruption to GnRH neuron activity are independent, suggesting that susceptibility to reproductive endocrine comorbidities is driven by other risk factors.

Female-Specific Pituitary Hypersensitivity to Gonadotropin-Releasing Hormone in a Mouse Model of Chronic Temporal Lobe Epilepsy

“…Although there were no differences observed in LH pulsatility or FSH levels, most IHKA females develop elongated estrous cycles (22)(23)(24)27). The present results suggest that altered gonadotropin levels may not contribute to the development of this phenotype.…”

“…Controls were injected with the same volume of saline at the same location. 14 days after injection, the mice were implanted with twisted bipolar EEG electrodes (P1 Technologies) into the ipsilateral hippocampus (22,24,28,29). Implantation was slightly dorsal and lateral to the injection location (from Bregma: 1.75 mm lateral, 1.25 mm ventral).…”

“…Previous studies have shown that 24 hours of EEG monitoring is an accurate approximation of overall seizure burden in IHKA mice (24). Therefore, to confirm the development of chronic epilepsy and to evaluate whether seizure burden in the 24-hour period prior to LH collection was correlated to pulsatile LH release patterns, we compared EEG signals to the respective LH parameters from each animal.…”

“…GnRH-Cre females and males (Jackson Labs #021207) on the C57BL/6J background were housed in a 14:10 h light:dark cycle (lights off at 1900 h) with food and water available ad libitum. The animals used in this study were progeny not used in breeding pairs for ongoing projects that require Cre expression in GnRH neurons (23)(24)(25). After weaning, all animals were group-housed with 2-5 mice per cage.…”

“…Additionally, multiple rodent models of epilepsy exhibit disrupted estrous cycles (18)(19)(20)(21)(22). In the intrahippocampal kainic acid (IHKA) mouse model of TLE, hypothalamic GnRH neurons show disrupted firing and excitability in a manner that not only changes over the estrous cycle, but is also associated with the severity of estrous cycle disruption (23,24). These effects are also reflected in changes in GABAergic transmission to GnRH neurons (25).…”